This invention relates to curable coating compositions, particularly to curable compositions utilizing a carbamate- or urea-functional compound as one of the components of the composition.
Curable coating compositions such as thermoset coatings are widely used in the coatings art. They are often used for topcoats in the automotive and industrial coatings industry. Color-plus-clear composite coatings are particularly useful as topcoats where exceptional gloss, depth of color, distinctness of image, or special metallic effects are desired. The automotive industry has made extensive use of these coatings for automotive body panels. Color-plus-clear composite coatings, however, require an extremely high degree of clarity in the clearcoat to achieve the desired visual effect. High-gloss coatings also require a low degree of visual aberations at the surface of the coating in order to achieve the desired visual effect such as high distinctness of image (DOI).
Such coatings are especially susceptible to a phenomenon known as environmental etch. Environmental etch manifests itself as spots or marks on or in the finish of the coating that often cannot be rubbed out.
Curable coating compositions based on curable components having carbamate or urea functionality have been proposed have been described in the art to provide etch-resistant coatings, e.g., U.S. Pat. No. 5,356,669 and WO 94/10211.
In addition to resistance to environmental etch, a number of other characteristics can be desireable. For example, it may be desireable to provide a coating having a high degree of flexibility. This can be particularly advantageous if the substrate on which the coating is placed is itself flexible, as in the case of plastic, leather, or textile substrates.
It is also desirable to reduce the amount of solvent required in coating compositions in order to reduce the volatile organic content (VOC), which is better for the environment.
Finally, it is desirable to provide options of different types of carbamate- or urea-functional materials to provide coatings with a good combination of properties such as durability, hardness, and resistance to scratching, marring, solvents, and acids.
According to the present invention, there is provided a coating composition comprising
(A) a carbamate- or urea-functional compound that is the reaction product of a mixture comprising
(1) a compound comprising a carbamate or urea group or a group that can be converted to carbamate or urea, and a hydroxyl functional group that is the reaction product of
(a) a compound comprising a carbamate or urea group or a group that can be converted to a carbamate or urea group, and an active hydrogen group that is reactive with a lactone ring or a hydroxy carboxylic acid, and
(b) a lactone or a hydroxy carboxylic acid, and
(2) a compound that is reactive with hydroxyl groups on a plurality of molecules of compound (A)(1), but that is not reactive with the carbamate or urea groups on compound (A)(1),
(B) a compound comprising a plurality of groups that are reactive with carbamate or urea.
The present invention provides coatings having a good combination of properties such as durability, hardness, and resistance to scratching, marring, solvents, and acids. Coating compositions according to the invention can also provide low VOC levels, and can be used to prepare coatings having good flexibility for use over flexible substrates.
According to the present invention, compound (A) has carbamate or urea functionality, and is formed by reaction of a compound (A)(1) having carbamate or urea groups or groups that can be converted to carbamate or urea and hydroxyl groups, with a compound (A)(2). Compound (A)(1) is the reaction product of a compound having at least one carbamate or urea group (or a group that can be converted to carbamate or urea) and an active hydrogen group (A)(1)(a) with a lactone or hydroxy carboxylic acid (A)(1)(b).
Carbamate groups can generally be characterized by the formula 
wherein R is H or alkyl, preferably of 1 to 4 carbon atoms. Preferably, R is H or methyl, and more preferably R is H. Urea groups can generally be characterized by the formula 
wherein Rxe2x80x2 and Rxe2x80x3 each independently represents H or alkyl, preferably of 1 to 4 carbon atoms, or Rxe2x80x2 and Rxe2x80x3 may together form a heterocyclic ring structure (e.g., where Rxe2x80x2 and Rxe2x80x3 form an ethylene bridge).
The compound (A)(1) can be formed by reacting a lactone or hydroxy carboxylic acid with a compound having an active hydrogen group capable of undergoing a condensation reaction with the acid group of the hydroxy carboxylic acid or ring-opening the lactone (e.g., hydroxyl, primary amine, acid) and a carbamate or urea group or a group that can be converted to carbamate or urea. When a compound having an active hydrogen group and a group that can be converted to carbamate or urea is used to react with the lactone or hydroxy carboxylic acid, conversion of the group to a carbamate or urea can be accomplished during or after the ring-opening reaction.
Compounds having a carbamate or urea group and an active hydrogen group are known in the art. Hydroxypropyl carbamate and hydroxyethyl ethylene urea, for example, are well known and commercially available. Amino carbamates are described in U.S. Pat. No. 2,842,523. Hydroxyl ureas may also be prepared by reacting an oxazolidone with ammonia or a primary amine or by reacting ethylene oxide with ammonia to form an amino alcohol and then reacting the amine group of that compound or any other amino alcohol with hydrochloric acid, then urea to form a hydroxy urea. Amino ureas can be prepared, for example, by reacting a ketone with a diamine having one amine group protected from reaction (e.g., by steric hindrance), followed by reaction with HNCO (i.e., the product of the thermal decomposition of urea), and then water. Alternatively, these compounds can be prepared by starting with a compound having an active hydrogen and a group that can be converted to carbamate or urea as described below, and then converting that group to the carbamate or urea prior to commencement of the reaction with the lactone or hydroxy carboxylic acid.
Groups that can be converted to carbamate include cyclic carbonate groups, epoxy groups, and unsaturated bonds. Cyclic carbonate groups can be converted to carbamate groups by reaction with ammonia or a primary amine, which ring-opens the cyclic carbonate to form a xcex2-hydroxy carbamate. Epoxy groups can be converted to carbamate groups by first converting to a cyclic carbonate group by reaction with CO2. This can be done at any pressure from atmospheric up to supercritical CO2 pressures, but is preferably under elevated pressure (e.g., 60-150 psi). The temperature for this reaction is preferably 60-150xc2x0 C. Useful catalysts include any that activate an oxirane ring, such as tertiary amine or quaternary salts (e.g., tetramethyl ammonium bromide), combinations of complex organotin halides and alkyl phosphonium halides (e.g., (CH3)3SnI, Bu4SnI, Bu4PI, and (CH3)4PI), potassium salts (e.g., K2CO3, KI) preferably in combination with crown ethers, tin octoate, calcium octoate, and the like. The cyclic carbonate group can then be converted to a carbamate group as described above. Any unsaturated bond can be converted to carbamate groups by first reacting with peroxide to convert to an epoxy group, then with CO2 to form a cyclic carbonate, and then with ammonia or a primary amine to form the carbamate.
Other groups, such as hydroxyl groups or isocyanate groups can also be converted to carbamate groups to form a compound (A)(1)(a). However, if such groups were to be present on the compound (A)(1)(a) and then converted to carbamate after reaction with the lactone or hydroxy carboxylic acid, they would have to be blocked so that they would not react with the lactone, the hydroxy carboxylic acid, or with other active hydrogen groups. When blocking these groups is not feasible, the conversion to carbamate or urea would have to be completed prior to reaction with the lactone or hydroxy carboxylic acid. Hydroxyl groups can be converted to carbamate groups by reaction with a monoisocyanate (e.g., methyl isocyanate) to form a secondary carbamate group or with cyanic acid (which may be formed in situ by thermal decomposition of urea) to form a primary carbamate group (i.e., unsubstituted carbamates). This reaction preferably occurs in the presence of a catalyst as is known in the art. A hydroxyl group can also be reacted with phosgene and then ammonia to form a compound having primary carbamate group(s), or by reaction of a hydroxyl with phosgene and then a primary amine to form a compound having secondary carbamate groups. Another approach is to react an isocyanate with a compound such as hydroxyalkyl carbamate to form a carbamate-capped isocyanate derivative. For example, one isocyanate group on toluene diisocyanate can be reacted with hydroxypropyl carbamate, followed by reaction of the other isocyanate group with an excess of polyol to form a hydroxy carbamate. Finally, carbamates can be prepared by a transesterification approach where hydroxyl group reacted with an alkyl carbamate (e.g., methyl carbamate, ethyl carbamate, butyl carbamate) to form a primary carbamate group-containing compound. This reaction is performed under heat, preferably in the presence of a catalyst such as an organometallic catalyst (e.g., dibutyltin dilaurate). Other techniques for preparing carbamates are also known in the art and are described, for example, in P. Adams and F. Baron, xe2x80x9cEsters of Carbamic Acidxe2x80x9d, Chemical Review, v. 65, 1965.
Groups such as oxazolidone can also be converted to urea after reaction with the lactone or hydroxy carboxylic acid. For example, hydroxyethyl oxazolidone can be used to initiate the reaction with the lactone or hydroxy carboxylic acid, followed by reaction of ammonia or a primary amine with the oxazolidone to generate the urea functional group.
Other groups, such as amino groups or isocyanate groups can also be converted to urea groups to form a compound (A)(1)(a). However, if such groups were to be present on the compound (A)(1)(a) and then converted to urea after reaction with the lactone or hydroxy carboxylic acid, they would have to be blocked so that they would not react with the lactone, the hydroxy carboxylic acid, or with other active hydrogen groups. When blocking these groups is not feasible, the conversion to carbamate or urea would have to be completed prior to the reaction with the lactone or hydroxy carboxylic acid. Amino groups can be converted to urea groups by reaction with a monoisocyanate (e.g., methyl isocyanate) to form a secondary urea group or with cyanic acid (which may be formed in situ by thermal decomposition of urea) to form a primary urea group. This reaction preferably occurs in the presence of a catalyst as is known in the art. An amino group can also be reacted with phosgene and then ammonia to form a compound having primary urea group(s), or by reaction of an amino group with phosgene and then a primary amine to form a compound having secondary urea groups. Another approach is to react an isocyanate with a hydroxy urea compound to form a urea-capped isocyanate derivative. For example, one isocyanate group on toluene diisocyanate can be reacted with hydroxyethyl ethylene urea, followed by reaction of the other isocyanate group with an excess of polyol to form a hydroxy carbamate.
One preferred class of compounds having an active hydrogen group and a group that can be converted to carbamate is the hydroxyalkyl cyclic carbonates. Hydroxyalkyl cyclic carbonates can be prepared by a number of approaches. Certain hydroxyalkyl cyclic carbonates like 3-hydroxypropyl carbonate (i.e., glycerine carbonate) are commercially available. Cyclic carbonate compounds can be synthesized by any of several different approaches. One approach involves reacting an epoxy group-containing compound with CO2, under conditions and with catalysts as described hereinabove. Epoxides can also be reacted with xcex2-butyrolactone in the presence of such catalysts. In another approach, a glycol like glycerine is reacted at temperatures of at least 80xc2x0 C. with diethyl carbonate in the presence of a catalyst (e.g., potassium carbonate) to form a hydroxyalkyl carbonate. Alternatively, a functional compound containing a ketal of a 1,2-diol having the structure: 
can be ring-opened with water, preferably with a trace amount of acid, to form a 1,2-glycol, which is then further reacted with diethyl carbonate to form the cyclic carbonate.
Cyclic carbonates typically have 5-6-membered rings, as is known in the art. Five-membered rings are preferred, due to their ease of synthesis and greater degree of commercial availability. Six-membered rings can be synthesized by reacting phosgene with 1,3-propane diol under conditions known in the art for the formation of cyclic carbonates. Preferred hydroxyalkyl cyclic carbonates used in the practice can be represented by the formula: 
where R (or each instance of R if n is more than 1) is a hydroxyalkyl group of 1-18 carbon atoms, preferably 1-6 carbon atoms, and more preferably 1-3 carbon atoms, which may be linear or branched and may have subsituents in addition to the hydroxyl (which itself may be primary, secondary, or tertiary), and n is 1 or 2, which may be substituted by one or more other substituents such as blocked amines or unsaturated groups. More preferably, R is xe2x80x94CmH2mOH where the hydroxyl may be primary or secondary and m is 1 to 8, and even more preferably, R is xe2x80x94(CH2)pxe2x80x94OH where the hydroxyl is primary and p is 1 to 2.
Lactones that can be ring opened by an active hydrogen are well-known in the art. They include, for example, xcex5-caprolactone, xcex3-caprolactone, xcex2-butyrolactone, xcex2-propriolactone, xcex3-butyrolactone, xcex1-methyl-xcex3-butyrolactone, xcex2-methyl-xcex3-butyrolactone, xcex3-valerolactone, xcex4-valerolactone, xcex3-nonanoic lactone, xcex3-octanoic lactone, and pentolactone. In one preferred embodiment, the lactone is xcex5-caprolactone. Lactones useful in the practice of the invention can also be characterized by the formula: 
wherein n is a positive integer of 1 to 7 and R is one or more H atoms, or substituted or unsubstituted alkyl groups of 1-7 carbon atoms.
The lactone ring-opening reaction is typically conducted under elevated temperature (e.g., 80-150xc2x0 C.). The reactants are usually liquids so a solvent is not necessary. However, a solvent may be useful in promoting good conditions for the reaction even if the reactants are liquid. Any non-reactive solvent may be used, including both polar and nonpolar organic solvents. Examples of useful solvents include toluene, xylene, methyl ethyl ketone, methyl isobutyl ketone, and the like. A catalyst is preferably present. Useful catalysts include proton acids (e.g., octanoic acid, Amberlyst(copyright) 15 (Rohm and Haas)), and tin catalysts (e.g., stannous octoate). Alternatively, the reaction can be initiated by forming a sodium salt of the hydroxyl group on the molecules to react with the lactone ring.
The lactone ring-opening reaction provides chain extension of the molecule if sufficient amounts of the lactone are present. The relative amounts of the carbamate or urea compound (A)(1)(a) and the lactone (A)(1)(b) can be varied to control the degree of chain extension. The opening of the lactone ring with a hydroxyl or amine group results in the formation of an ester or amide and an OH group. The OH group can then react with another available lactone ring, thus resulting in chain extension. The reaction is thus controlled by the proportion of lactone in the relative to the amount of initiator compound (A)(1)(a). In the practice of the present invention, the ratio of equivalents of lactone from (A)(1)(b) to equivalents of active hydrogen groups on (A)(1)(a) is preferably from 0.1:1 to 10:1, and more preferably from 1:1 to 5:1. When the lactone is opened with with an acid, the resulting compound has an acid group, which can then be converted to a hydroxyl group by well-known techniques such as reaction with ethylene oxide.
A compound (A)(1) having a hydroxyl active hydrogen group can also be reacted with a hydroxy carboxylic acid to form the carbamate- or urea-functional compound (A). Useful hydroxy carboxylic acids include dimethylhydroxypropionic acid, hydroxy stearic acid, tartaric acid, lactic acid, 2-hydroxyethyl benzoic acid, and N-(2-hydroxyethyl)ethylene diamine triacetic acid. The reaction can be conducted under typical transesterification conditions, e.g., temperatures from room temperature to 150xc2x0 C. with transesterification catalysts such as such as calcium octoate, metal hydroxides (e.g., KOH), Group I or II metals (e.g., Na, Li), metal carbonates (e.g., K2CO3) which may be enhanced by use in combination with crown ethers, metal oxides (e.g., dibutyltin oxide), metal alkoxides (e.g., NaOCH3, Al(OC3H7)3), metal esters (e.g., stannous octoate, calcium octoate, or protic acids (e.g., H2SO4), MgCO3, or Ph4SbI. The reaction may also be conducted at room temperature with a polymer-supported catalyst such as Amberlyst-15(copyright) (Rohm and Haas) as described by R. Anand, Synthetic Communications, 24(19), 2743-47 (1994), the disclosure of which is incorporated herein by reference.
The compound (A) is formed by reaction of the compound (A)(1) with the compound (A)(2). Compound (A)(2) is reactive with the hydroxyl groups on a plurality of molecules of compound (A)(1), but not reactive with the carbamate or urea groups on compound (A)(1). Thus, in the final product, the residue of compound (A)(2) can be described as a core to which a plurality of carbamate- or urea-functional residues of compound (A)(1) are attached. It is also contemplated that compound (A)(1) may be admixed with other compounds comprising at least one hydroxyl group plus a carbamate or urea group (e.g., hydroxypropyl carbamate) prior to the reaction with compound (A)(2). In such a case, the resulting reaction product mixture will reflect the stoichiometric ratio of compound (A)(1) to such other compounds.
Compounds that are useful as (A)(2) include polyisocyanates, dialkyl carbonates, cyclic carbonates, CO2, phosgene, acetals, cyclic or linear phosphazene-based compounds, substituted or unsubstituted cyclic siloxanes or silanes, or substituted or unsubstituted linear siloxanes or silanes, which may be described by the formula SiXmRn where X is a group that is reactive with protons, such as a halide, alkoxy, hydride, or acetate, R is a group that is non-reactive with protons such as alkyl, silane, or siloxane, m=2-4, and m+n=4, SO2 sources such as SO3 or SO2Cl2, POCl3, POCl2R where R is alkyl or aryl. With certain of the compounds (A)(2), a diol may also be included in the reaction mixture comprising (A)(1) and (A)(2) to obtain chain extension with carbamate or urea termination. This can be done, for example, with phosgene where the phosgene/diol reaction results in chain extension and the reaction of phosgene with compound (A)(1) results in chain termination with a carbamate or urea group.
The polyisocyanate can be an aliphatic polyisocyanate, including a cycloaliphatic polyisocyanate or an aromatic polyisocyanate. Useful aliphatic polyisocyanates include aliphatic diisocyanates such as ethylene diisocyanate, 1,2-diisocyanatopropane, 1,3-diisocyanatopropane, 1,6-diisocyanatohexane, 1,4-butylene diisocyanate, lysine diisocyanate, 1,4-methylene bis-(cyclohexyl isocyanate) and isophorone diisocyanate. Useful aromatic diisocyanates and araliphatic diisocyanates include the various isomers of toluene diisocyanate, meta-xylylenediioscyanate and para-xylylenediisocyanate, also 4-chloro-1,3-phenylene diisocyanate, 1,5-tetrahydro-naphthalene diisocyanate, 4,4xe2x80x2-dibenzyl diisocyanate and 1,2,4-benzene triisocyanate can be used. In addition, the various isomers of xcex1,xcex1,xcex1xe2x80x2,xcex1xe2x80x2-tetramethyl xylylene diisocyanate can be used. Oligomeric or polymeric polyisocyanates prepared by reaction of an excess of monomeric polyisocyanates with a polyol may be used. Also, isocyanurates such as the isocyanurate of isophorone diisocyanate or the isocyanurate of hexamethylene diisocyanate may be used. Biurets of isocyanates such as DESMODUR(copyright) N100 from Mobay may also be useful.
Dialkyl carbonates, cyclic carbonates, CO2, diphenyl carbonates, or phosgene may be used as compound (A)(2) to react with and link two (A)(1) compounds via a carbonate linking group. When phosgene is used, phosgene may be added to a solution of compound (A)(1) at a molar ratio of about 1 mole phosgene to 2 moles (A)(1) (or 2 moles (A)(1) plus other hydroxy carbamate or urea compounds such as hydroxypropyl carbamate). This reaction may be conducted at temperatures of less than 7xc2x0 C. or under pressure in order to maintain phosgene in it""s liquid state, or alternatively, gaseous phosgene may be bubbled through the system. A salting base (e.g., NaOH) may be used to help drive the reaction. The reaction may be conducted in virtually any aprotic solvent at temperatures of xe2x88x9220xc2x0 C. to 80xc2x0 C. and pressures of atmospheric to 40 psi.
Cyclic carbonates or dialkyl carbonates may be used as compound (A)(2) to react with compound (A)(1) by heating (e.g., 80-200xc2x0 C.) the appropriate molar mixture (2 moles (A)(1) plus any other hydroxy carbamate or urea and 1 mole cyclic carbonate or dialkyl carbonate) with a transesterification catalyst such as calcium octoate. Useful dialkyl carbonates include diethyl carbonate, dimethyl carbonate, dipropyl carbonate, diphenyl carbonate, and dibutyl carbonate. Useful cyclic carbonates include propylene carbonate, glycerine carbonate, and dimethyl ethylene carbonate. Cyclic carbonates may also be formed from any unsaturated bond by reaction of the unsaturated bond with peroxide to form an oxirane ring, followed by reaction with CO2 to form the cyclic carbonate. Useful catalysts include metal hydroxides (e.g., KOH), Group I or II metals (e.g., Na, Li), metal carbonates (e.g., K2CO3) which may be enhanced by use in combination with crown ethers, metal oxides (e.g., dibutyltin oxide), metal alkoxides (e.g., NaOCH3, Al(OC3H7)3), metal esters (e.g., stannous octoate, calcium octoate), or protic acids (e.g., H2SO4), MgCO3, or Ph4SbI. Any solvents used should be inert to transesterification. The catalysts and/or reaction conditions may need to be adjusted to minimize transesterification of the ester groups in compound (A)(1). CO2 may also be used as compound (A)(2) under similar conditions with similar catalysts plus it may be used at pressures of 1 to 40 atm.
Compounds having inorganic reactive groups may also be used to react with the hydroxyl groups of compound (A)(1). These include phosphorus compounds such as POCl3 or hexachlorocyclotriphosphazene, SO2 sources such as SO3 or SO2Cl2 or silane-based systems such as substituted or unsubstituted cyclic siloxanes or silanes, or substituted or unsubstituted linear siloxanes or silanes, which may be described by the formula SiXmRn where X is a group that is reactive with protons, such as a halide, alkoxy, hydride, or acetate, R is a group that is non-reactive with protons such as alkyl, silane, or siloxane, m=2-4, and m+n=4.
Phosphorus-containing compounds such as phosphazene-based compounds (e.g., hexachlorocyclotriphosphazene) or POCl3 may be used as compound (A)(2) to react with (A)(1). In a typical reaction, one equivalent (based on chlorine content) of the phosphorus reagent is dissolved in a dry ether solvent such as diethyl ether of tetrahydrofuran to form a solution of approximately 50%. 1.5 equivalents of sodium hydride are added followed by one equivalent of (A)(1) (or (A)(1) plus other hydroxy carbamate or urea compounds). The mixture is allowed to exotherm to the reflux temperature of the solvent, with the reaction temperature controlled by the addition rate of the (A)(1) compound. After addition of the (A)(1) compound is complete, the reaction mixture is heated to reflux and held for 2-3 hours. The mixture is then cooled, filtered to remove sodium chloride and any unreacted sodium hydride, and the solvent removed under vacuum.
Silane-based compounds may also be used as compound (A)(2). Such compounds may be described by the formula SiXmRn where X is a group that is reactive with protons, such as a halide, alkoxy, hydride, or acetate, R is a group that is non-reactive with protons such as alkyl, silane, or siloxane, m=2-4, and m+n=4. These compounds may react with (A)(1) in any dry aprotic solvent (e.g., tetrahydrofuran) under conditions known in the art, which may depend on the nature of the X group. When X is a hydride, the reaction is preferably begun with chilled reactants (e.g., 0xc2x0 C.) under an inert atmosphere using catalysts such as tin catalysts. After the addition of materials is complete, amd dry methanol is added to react with any free remaining Si-H bonds. If X is a halide, the reaction is preferably begun under an inert atmosphere at room temperature. The mixture is then heated to reflux to drive the reaction to completion. HCl is given off as a by-product. If X is alkoxy, the reaction is preferably begun under an inert atmosphere at room temperature, which may be maintained for the duration of the reaction. A molecular sieve may be used to absorb the alcohol side product that is formed. Slightly basic or acidic pH will accelerate this reaction; however, it will also accelerate the formation of Sixe2x80x94Oxe2x80x94Si bonds.
For SO2 sources, the SO3 can be reacted with the (A)(1) by bubbling SO3 through the (A)(1) compound if it is in liquid form or by dissolving (A)(1) in a solvent and then bubbling S03 through the solution. The reaction of SO2Cl2 with (A) (1) may be assisted by the pre-reaction of (A)(1) with Na or NaOR (where R is an organic radical).
The composition of the invention is cured by a reaction of the carbamate- or urea-functional compound (A) with a component (B) that is a compound having a plurality of functional groups that are reactive with the carbamate or urea groups on component (A). Such reactive groups include active methylol or methylalkoxy groups on aminoplast crosslinking agents or on other compounds such as phenol/formaldehyde adducts, siloxane or silane groups, and anhydride groups. Examples of (B) compounds include melamine formaldehyde resin (including monomeric or polymeric melamine resin and partially or fully alkylated melamine resin), urea resins (e.g., methylol ureas such as urea formaldehyde resin, alkoxy ureas such as butylated urea formaldehyde resin), N-methylol acrylamide emulsions, isobutoxy methyl acrylamide emulsions, polyanhydrides (e.g., polysuccinic anhydride), and siloxanes or silanes (e.g., dimethyldimethoxy silane). Aminoplast resin such as melamine formaldehyde resin or urea formaldehyde resin are especially preferred. Also preferred are aminoplast resins where one or more of the amino nitrogens is substituted with a carbamate group for use in a process with a curing temperature below 150xc2x0 C., as described in U.S. Pat. No. 5,300,328.
A solvent may optionally be utilized in the coating composition used in the practice of the present invention. The coating composition according to the present invention can be applied without solvent, especially if the degree of chain extension for component (A) is limited. However, in many cases, it is desirable to use a solvent in the coating composition as well. This solvent should act as a solvent with respect to both the carbamate- or urea-functional compound (A) as well as the component (B). In general, depending on the solubility characteristics of components (A) and (B), the solvent can be any organic solvent and/or water. In one preferred embodiment, the solvent is a polar organic solvent. More preferably, the solvent is a polar aliphatic solvents or polar aromatic solvents. Still more preferably, the solvent is a ketone, ester, acetate, aprotic amide, aprotic sulfoxide, or aprotic amine. Examples of useful solvents include methyl ethyl ketone, methyl isobutyl ketone, amyl acetate, ethylene glycol butyl ether-acetate, propylene glycol monomethyl ether acetate, xylene, N-methylpyrrolidone, or blends of aromatic hydrocarbons. In another embodiment, the solvent can be water or a mixture of water with co-solvents.
The coating composition used in the practice of the invention may include a catalyst to enhance the cure reaction. For example, when aminoplast compounds, especially monomeric melamines, are used as component (B), a strong acid catalyst may be utilized to enhance the cure reaction. Such catalysts are well-known in the art and include, for example, p-toluenesulfonic acid, dinonylnaphthalene disulfonic acid, dodecylbenzenesulfonic acid, phenyl acid phosphate, monobutyl maleate, butyl phosphate, and hydroxy phosphate ester. Other catalysts that may be useful in the composition of the invention include Lewis acids, zinc salts, and tin salts.
Although a solvent may be present in the coating composition in an amount of from about 0.01 weight percent to about 99 weight percent, it is preferably present in an amount of less than 35%, more preferably less than 25% and most preferably less than 15%. The coating composition preferably has a VOC (VOC is defined herein as VOC according to ASTM D3960) of less than 3.5 lbs/gal, more preferably less than 2.5 lbs/gal, and most preferably less than 1.5 lbs/gal.
Coating compositions can be coated on the article by any of a number of techniques well-known in the art. These include, for example, spray coating, dip coating, roll coating, curtain coating, and the like. For automotive body panels, spray coating is preferred. One advantage that can be achieved with coating compositions according to the invention is that coatings with a high degree of flexibility can be prepared. Accordingly, in a preferred embodiment, the substrate onto which the coating is applied is flexible, such as plastic, leather, or textile substrates.
Any additional agent used, for example, surfactants, fillers, stabilizers, wetting agents, dispersing agents, adhesion promoters, UV absorbers, HALS, etc. may be incorporated into the coating composition. While the agents are well-known in the prior art, the amount used must be controlled to avoid adversely affecting the coating characteristics.
In one preferred embodiment, the coating composition according to the invention is preferably utilized in a high-gloss coating and/or as the clearcoat of a composite color-plus-clear coating. High-gloss coatings as used herein are coatings having a 20xc2x0 gloss (ASTM D523-89) or a DOI (ASTM E430-91) of at least 80. In other preferred embodiments, the coating composition may be utilized to prepare high-gloss or low-gloss primer or enamel coatings.
When the coating composition of the invention is used as a high-gloss pigmented paint coating, the pigment may be any organic or inorganic compounds or colored materials, fillers, metallic or other inorganic flake materials such as mica or aluminum flake, and other materials of kind that the art normally names as pigments. Pigments are usually used in the composition in an amount of 2% to 350%, based on the total weight (not including solvent) of components A and B (i.e., a P:B ratio of 0.02 to 3.5).
When the coating composition according to the invention is used as the clearcoat of a composite color-plus-clear coating, the pigmented basecoat composition may any of a number of types well-known in the art, and does not require explanation in detail herein. Polymers known in the art to be useful in basecoat compositions include acrylics, vinyls, polyurethanes, polycarbonates, polyesters, alkyds, and siloxanes. Preferred polymers include acrylics and polyurethanes. In one preferred embodiment of the invention, the basecoat composition also utilizes a carbamate-functional acrylic polymer. Basecoat polymers are preferably crosslinkable, and thus comprise one or more type of cross-linkable functional groups. Such groups include, for example, hydroxy, isocyanate, amine, epoxy, acrylate, vinyl, silane, and acetoacetate groups. These groups may be masked or blocked in such a way so that they are unblocked and available for the cross-linking reaction under the desired curing conditions, generally elevated temperatures. Useful cross-linkable functional groups include hydroxy, epoxy, acid, anhydride, silane, and acetoacetate groups. Preferred cross-linkable functional groups include hydroxy functional groups and amino functional groups.
Basecoat polymers may be self-cross-linkable, or may require a separate cross-linking agent that is reactive with the functional groups of the polymer. When the polymer comprises hydroxy functional groups, for example, the cross-linking agent may be an aminoplast resin, isocyanate and blocked isocyanates (including isocyanurates), and acid or anhydride functional cross-linking agents.
The coating compositions described herein are preferably subjected to conditions so as to cure the coating layers. Although various methods of curing may be used, heat-curing is preferred. Generally, heat curing is effected by exposing the coated article to elevated temperatures provided primarily by radiative heat sources. Curing temperatures will vary depending on the particular blocking groups used in the cross-linking agents, however they generally range between 93xc2x0 C. and 177xc2x0 C. The coating composition according to the present invention is curable even at relatively low cure temperatures. Thus, in a preferred embodiment, the cure temperature is preferably between 115xc2x0 C. and 150xc2x0 C., and more preferably at temperatures between 115xc2x0 C. and 138xc2x0 C. for a blocked acid catalyzed system. For an unblocked acid catalyzed system, the cure temperature is preferably between 82xc2x0 C. and 99xc2x0 C. The curing time will vary depending on the particular components used, and physical parameters such as the thickness of the layers, however, typical curing times range from 15 to 60 minutes, and preferably 15-25 minutes for blocked acid catalyzed systems and 10-20 minutes for unblocked acid catalyzed systems.